Multilevel interconnection board and method of fabricating the same

ABSTRACT

A stack of 50 layers of a first pitch-base carbon fiber sheet is formed, two sets of stack each having two second pitch-base carbon fiber sheets stacked therein are fabricated. At this time, the second carbon fiber sheets have a thermal expansion coefficient larger than that of the first carbon fiber sheet. Next, the stack of the first carbon fiber sheet is then held between two sets of stack of the second carbon fiber sheets. The stack of the first and second carbon fiber sheets are then impregnated with an epoxy-base resin composition and the resin is solidified. As a result a prepreg composed of the first and second carbon fiber sheets and the resin component composed of the epoxy-base resin composition is obtained. Thereafter, interconnections and the like are then formed on the prepreg, to thereby complete a multilevel interconnection board.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/339,511, filed on Jan. 26, 2006, which is based upon and claims thebenefit of priority from the prior Japanese Patent Application No.2005-290357, filed on Oct. 3, 2005, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multilevel interconnection board usedfor testing semiconductor devices and so forth, and a method offabricating the same.

2. Description of the Related Art

Tester board (multilevel interconnection board) is used typically inelectrical tests of wafers having semiconductor devices formed thereon,in order to make contact between probes and terminals of eachsemiconductor device. A high level of dimensional accuracy is requiredfor the multilevel interconnection board, because the probes andterminals are disposed at extremely narrow intervals. With recentincrease in size of wafers, an extremely high dimensional accuracy hasbeen required for the multilevel interconnection board applied inparticular to 300-mm-diameter wafers. In some cases, the multilevelinterconnection board is used also under high temperatures. The corebase and/or top layer of the multilevel interconnection board aretherefore configured using a material having a small thermal expansioncoefficient, which is less likely to deform even under hightemperatures. Examples of multilevel interconnection board ever put intopractical use include those having core base composed of Invar(Fe—Ni-base alloy), Covar (Fe—Ni—Co-base alloy) andcarbon-fiber-reinforced plastics (CFRP).

A known structure adopted to the multilevel interconnection board issuch as having the thermal expansion coefficient α which decreases inthe direction from the core base to the surficial portion. An exemplarystructure ever adopted is such as having a carbon-fiber-impregnatedresin component (α=1) held between glass-fiber-impregnated resincomponents (α=15), and the stack is further held between thin-filmmultilevel resin components (α=20). Adoption of this structure makes itpossible to reduce difference between thermal expansion coefficients atthe interface between the different materials.

The multilevel interconnection board may, however, be suffered fromwarping or cracking even if the above-described structure is adopted,due to internal stress concentrated at the interface at a temperature ashigh as around 150° C. The board may also cause short-circuiting due toimpregnation of copper plating solution in the process of forminginterconnections in through-holes. This consequently makes the boardunavailable for reliability test under high-temperature, high-humidityconditions.

Related arts are disclosed in a patent document 1 (Japanese PatentApplication Laid-Open No. 2003-273482), a patent document 2 (JapanesePatent Application Laid-Open No. 3-104191), and a patent document 3(Japanese Patent Application Laid-Open No. 5-286776).

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide amultilevel interconnection board capable of suppressing dimensionalchanges under high temperatures, and a method of fabricating the same.

The multilevel interconnection board according to the present inventionhas a base, and a plurality of interconnection layers formed on thebase. The base is provided with a first fiber group, and a second fibergroup. The second fiber group is disposed closer to the interconnectionlayers than the first fiber group. The second fiber group has a thermalexpansion coefficient larger than that of the first fiber group.

According to the method of fabricating the multilevel interconnectionboard of the present invention, a base is formed by stacking a firstfiber group and a second fiber group having a thermal expansioncoefficient larger than that of the first fiber group, and a pluralityof interconnection layers are then formed on the second fiber group ofthe base.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to FIG. 1L are sectional views sequentially showing a method offabricating a multilevel interconnection board according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Paragraphs below will specifically explain embodiments of the presentinvention, referring to the attached drawings. It is to be noted that astructure of the multilevel interconnection board will be described inconjunction with a method of fabrication of the same for theconvenience's sake. FIGS. 1A to 1L are sectional views sequentiallyshowing process steps of the method of fabrication of the multilevelinterconnection board according to the embodiment of the presentinvention.

In this embodiment, a stack (first carbon fiber group) having stackedtherein 50 pitch-base, plain-woven carbon fiber sheets 1, each having anareal weight of carbon fiber of 300 g/m² and a tensile elastic constantof 600 GPa, is formed. Two sets of stack (second carbon fiber group),each having stacked therein two pitch-base unidirectionally-woven carbonfiber sheets 2, for example, having an areal weight of carbon fiber of100 g/m² and a tensile elastic constant of 400 GPa, are formed. Theareal weight of carbon fiber of the carbon fiber sheets 2 herein is setsmaller than that of the carbon fiber sheet 1, because there is a demandof setting the thermal expansion coefficient of the carbon fiber sheet 2larger than that of the carbon fiber sheet 1 in the present embodiment.Next, as shown in FIG. 1A, the stack of the carbon fiber sheets 1 isheld between two sets of stacks of the carbon fiber sheet 2.

Next, as shown in FIG. 1B, an epoxy-base resin composition, for example,is impregnated into the stack of the carbon fiber sheets 1 and 2, andthe resin is then allowed to solidify. A prepreg 4 containing the carbonfiber sheets 1 and 2 and the resin component 3 composed of theepoxy-base resin composition is thus formed. The prepreg 4 is, forexample, 1.0 mm in thickness, and has a planar geometry of a 340 mm×510mm rectangle. The epoxy-base resin composition is preliminarily mixed,for example, with an alumina filler and a silica filler. The each amountof the alumina filler and the silica filler is 10% by weight of thewhole composition. The alumina filler used herein has a mean particlesize of 7 μm or less, and a thermal expansion coefficient of 7 ppm/K.The silica filler used herein has a mean particle size of 3 μm or less,and a thermal expansion coefficient of 3 ppm/K.

Thus formed prepreg 4 has, for example, a mean thermal expansioncoefficient in the thickness-direction (the Z direction) of 80 ppm/K oraround, and a mean thermal expansion coefficient in the direction inparallel with the surface (the X direction and the Y direction) of 2ppm/K or around. The mean thermal expansion coefficients herein are meanvalues of the thermal expansion coefficients within the temperaturerange from 25° C. to 200° C. The fillers are mixed herein for thepurpose of approximating the thermal expansion coefficient of the resincomponent 3 to the thermal expansion coefficient of a metal materialcomposing the interconnections, such as Cu. For an exemplary case whereCu interconnections are used, the thermal expansion coefficient thereofis 17 ppm/K, whereas the thermal expansion coefficient of the resincomponent 3 solely composed of the epoxy-base resin composition falls inthe range from 30 ppm/K to 60 ppm/K. That is why the alumina fillerand/or the silica filler, having the thermal expansion coefficientssmaller than that of Cu, are allowed to diffuse into the epoxy-baseresin composition so as to adjust the thermal expansion coefficient.

Thereafter, as shown in FIG. 1C, approximately 1,000 through-holes 5 of0.5 mm in diameter are formed in the prepreg 4 using a drill. There areno special limitations on the number of formation, position, andgeometry of the through-holes, allowing determination of theseparameters considering design information on the interconnection layout.

Next, as shown in FIG. 1D, insulating resin sheets 6 are stuck on bothsurfaces of the prepreg 4. The insulating resin sheets 6 used hereinare, for example, composed of a thermoplastic polyimide sheet. Theinsulating resin sheets 6 are stuck, for example, using a vacuumlaminator at a temperature of 120° C., and under a pressure of 0.1 MPa.

Next, the stack is subjected to vacuum pressing at 200° C. for 30minutes so as to fill the insulating resin component into thethrough-holes 5, as shown in FIG. 1E. The thickness of the portion ofthe prepreg 4 remaining on both sides is set to 0.05 mm. It is alsoallowable to fill the insulating resin component by using a knownlaminator, vacuum laminator or laminate pressing apparatus.

Next, as shown in FIG. 1F, a through-hole 7 of 0.2 mm in diameter isformed at the center of the insulating resin component filled in each ofthe through-holes 5 using a UV-YAG laser. It is also allowable to formthe through-holes 7 by using a carbon dioxide gas laser or an excimerlaser. It is also allowable to form them by plasma-assisted dry etchingprocess. It is still also allowable to mechanically form them using adrill.

The prepreg 4 is then subjected to desmearing using permanganic acid forthe purpose of removing resin residue and the like, and of modifying thesurface thereof. A catalyst is then applied over the entire surface, andan electroless plated copper film is then formed as a seed layer 8 overthe entire surface, as shown in FIG. 1G. Further thereon, a dry-typeresist film is stuck. The dry-type resist film herein is stuck, forexample, using a vacuum laminator at a temperature of 110° C. and undera pressure of 0.1 MPa. The seed layer 8 is formed also in thethrough-holes 7. After completion of the sticking, the dry-type resistfilm is then patterned to obtain a shape of interconnections. In thepatterning, a portion of the dry-type resist film where theinterconnection will be formed later is removed, for example, by aphotolithographic process. The seed layer 8 is then patterned by etchingthrough the patterned dry-type resist film as a mask, to thereby obtainthe shape of the interconnection pattern. An etching solution applicableherein is, for example, a mixed solution of hydrogen peroxide andsulfuric acid. The dry-type resist film is then removed. The dry-typeresist film can be removed simply by immersing the dry-type resist filmand the like into a sodium-hydroxide-base solution at 60° C. or aroundfor 5 minutes. It is also allowable to ash the resist film by oxygenashing.

Next, an electroplated copper film is formed on the seed layer 8 so asto form interconnections 9, as shown in FIG. 1H. It is to be noted thatthe interconnections 9 are formed also in the through-holes 7, becausethe seed layer 8 already resides also in the through-holes 7 prior tothe formation of the electroless plated copper film.

Next, as shown in FIG. 1I, an insulating resin sheets 10 composed ofphoto-sensitive polyimide, for example, are stuck on theinterconnections 9 under pressing. Next, as shown in FIG. 1J, holes 11are formed in the insulating resin sheets 10 by a photolithographicprocess, so as to reach the interconnections 9.

Thereafter, a catalyst is applied over the entire surface, and as shownin FIG. 1K, another cycle of formation and processing of the seed layer8 composed of an electroless plated copper film, and formation of theinterconnections 9 composed of an electroplated copper film are carriedout. Process steps from the formation of the insulating resin sheets 10up to the formation of the interconnections 9 are repeated thereafter,to thereby form five interconnection layers in total respectively onboth of the two surfaces, as shown in FIG. 1L. An overcoat layer issucceedingly formed by a combined process of a screen printing processand a photolithographic process, to thereby complete the multilevelinterconnection board.

In thus fabricated multilevel interconnection board, the thermalexpansion coefficient of the carbon fiber sheet 2 composing the prepreg4 as the base is larger than the thermal expansion coefficient of thecarbon fiber sheet 1. As a consequence, the thermal expansioncoefficient of the prepreg 4 gradually increases in thethickness-direction from the center portion towards the surface. Thethermal expansion coefficient of the surficial layers composed of theseed layers 8, the interconnections 9 and the insulating resin sheets 10is larger than that of the prepreg 4. This embodiment therefore makes itpossible to further moderate difference between the thermal expansioncoefficients between the prepreg 4 and the surficial layers. Morespecifically, the conventional structure, in which the thermal expansioncoefficients differ by 15 ppm/K or more between the core base and thelayer formed thereon, may raise internal stress ascribable to thedifference in the thermal expansion coefficient, whereas this embodimentcan suppress such unfavorable phenomenon. This makes it possible tofurther suppress the dimensional variation even if the board is usedunder a high-temperature, high-humidity conditions, so that the board ispreferably adopted to reliability tests of wafers.

The present inventors practically tested the multilevel interconnectionboard, fabricated as described in the above embodiment, and obtainedresults as below. In this test, a heat cycle of keeping the board at−65° C. for 30 minutes and then at +125° C. for 30 minutes was repeated1,000 times. Ratio of change of the contact resistance was found to be+5% or less. No peeling-off and crack were observed in the vicinity ofthe through-holes 7 and the like. There was also no observation ofshort-circuiting ascribable to impregnation of the copper platingsolution used for forming the interconnections 9 in the through-holes 7.

On the other hand, the same test made on a multilevel interconnectionboard fabricated by the conventional method showed short-circuiting atthe center portion of the through-holes.

As the carbon fiber, it is also allowable to use PAN(poly-acrylonitrile)-base carbon fiber, short carbon fiber, andnon-woven-fabric-base carbon fiber. For the case where the PAN-basecarbon fiber is used, it is allowable to use, for example, a plain-wovencarbon fiber sheet having an areal weight of carbon fiber of 450 g/m²and a tensile elastic constant of 500 GPa as the carbon fiber sheet 1,and to use a unidirectionally-woven carbon fiber sheet having an arealweight of carbon fiber of 100 g/m² and a tensile elastic constant of 300GPa as the carbon fiber sheet 2. The same test practically made by thepresent inventors on a multilevel interconnection board fabricated usingthese materials showed results similar to those obtained in theabove-described embodiment.

As a fiber group composing the prepreg (base), it is also allowable touse a glass fiber group, beside the carbon fiber group.

The thermal expansion coefficient is adjustable also by a method otherthan the adjustment of the areal weight of carbon fiber. For example, itis also allowable to set the thermal expansion coefficient of the carbonfiber composing the stack of the carbon fiber sheets on the surficiallayer side (second carbon fiber group) larger than the thermal expansioncoefficient of the carbon fiber composing the stack of the carbon fibersheets in the center portion (first carbon fiber group). It is stillalso allowable to set the fiber density per unit volume of the secondcarbon fiber group smaller than the fiber density of the first carbonfiber group. This is because the larger the fiber density of the carbonfiber group becomes, the larger the carbon fiber content per unit areabecomes, and this consequently increases the tensile elastic constant ofthe carbon fiber group, and reduces the thermal expansion coefficient.The same will apply also to the case where a glass fiber group is used.It is still also allowable to set the carbon purity of the second carbonfiber group smaller than the carbon purity of the first carbon fibergroup. This is because the higher the carbon purity becomes, the morereadily can intrinsic characteristics of carbon be obtained, and thisconsequently raises the elastic constant, and reduces the thermalexpansion coefficient.

The insulating resin sheet 10 composing the surficial layer may beprovided with variation in the characteristics such as thermal expansioncoefficient and elastic constant. For example, it is preferable to setthe thermal expansion coefficient of the insulating resin sheet 10 incontact with the prepreg 4 smaller than the thermal expansioncoefficient of the insulating resin sheet 10 disposed thereon or above.In other words, it is preferable to minimize the thermal expansioncoefficient of the insulating resin sheet 10 at the portion in contactwith the prepreg 4. This is because the provision of such variationmakes it possible to further moderate the changes in the thermalexpansion coefficient over the entire range of the multilevelinterconnection board. Also the thermal expansion coefficient of theinsulating resin sheet 10 is adjustable by varying, for example, thedensity of the fiber to be contained therein. Also with respect to thecharacteristics such as elastic constant, adjustment of thecharacteristics of the insulating resin sheet 10 in contact with theprepreg 4 to an intermediate level of those of the other upperinsulating resin sheets 10 makes it possible to reduce the stress whichgenerates in the vicinity of the interface between the prepreg 4 and thesurficial layer.

Although the above-described embodiment dealt with the prepreg 4 of 1.0mm thick, composed of the stack of 50 carbon fiber sheets 1 and twocarbon fiber sheets 2 holding it in between, it is also allowable tostack a plurality of thin prepregs. An exemplary case is such that astack of 10 carbon fiber sheets 1 is held between two stacks of thecarbon fiber sheets 2 to thereby form a prepreg of 0.2 mm thick. Fiveunits of thus-configured prepregs are then stacked and pressed in avacuum, to thereby form a prepreg of 1.0 mm thick. It is to be notedthat, even in the case of using such prepreg, the thermal expansioncoefficient of the first carbon fiber group disposed at the centerportion is set larger than those of the second carbon fiber groupsdisposed on the surficial layer sides.

The patent document 1 describes a core layer which adopts a stackedstructure of carbon fiber layers and insulating resin layers. There is,however, no description in relation to the thermal expansioncoefficient.

The patent document 2 describes adjustment of the thermal expansioncoefficient of a printed wiring board on which electronic components aremounted. There is, however, no description in relation to the multilevelinterconnection board.

The patent document 3 describes a board for semiconductor mountinghaving an intermediate layer provided between a metal layer and aceramic layer, wherein the ratio of content of metal of the intermediatelayer is gradated. There is, however, no description in relation to themultilevel interconnection board, and in relation to the thermalexpansion coefficient.

Because difference in the thermal expansion coefficients among portionsof the board can be moderated, the present invention makes it possibleto suppress generation of internal stress with increase in thetemperature, while suppressing the thermal expansion coefficient to alow level. This consequently improves the reliability and yield ratio ofthe board.

1. A method of fabricating a multilevel interconnection boardcomprising: forming a base by stacking a first fiber group, and a secondfiber group having a thermal expansion coefficient larger than that ofsaid first fiber group; and forming a plurality of interconnectionlayers on said second fiber group of said base, wherein the forming saidbase comprises impregnating a resin component into the first fiber groupand the second fiber group after the stacking, wherein said first andsaid second fiber groups are composed of a carbon fiber group, and afiber group having a carbon purity smaller than a carbon purity of saidfirst fiber group is used as said second fiber group.
 2. The method offabricating a multilevel interconnection board according to claim 1,wherein in the forming of said base, said second fiber groups aredisposed on both sides of said first fiber group, and said plurality ofinterconnection layers are formed on both sides of said base.
 3. Themethod of fabricating a multilevel interconnection board according toclaim 1, wherein a fiber group composed of fibers having a thermalexpansion coefficient larger than a thermal expansion coefficient offibers composing said first fiber group is used as said second fibergroup.
 4. The method of fabricating a multilevel interconnection boardaccording to claim 1, wherein a fiber group having a fiber density perunit volume smaller than a fiber density of said first fiber group isused as said second fiber group.
 5. The method of fabricating amultilevel interconnection board according to claim 1, wherein saidfirst and said second fiber groups are composed of any one selected fromthe group consisting of pitch-base carbon fiber, PAN-base carbon fiber,short carbon fiber and non-woven-fabric-base carbon fiber.
 6. The methodof fabricating a multilevel interconnection board according to claim 1,wherein the forming of said interconnection layers comprises forming aninsulating layer between two of said plurality of interconnectionlayers, said insulating layer having a thermal expansion coefficientminimized at the portion thereof in contact with said base.